Study on the established customized limits for the daily quality assurance procedure

Area of Science:

• Medical Physics
• Quality Assurance
• Statistical Analysis

Background:

• Current daily quality assurance (QA) protocols often lack specific limits in TG-142, leading to the use of monthly/annual limits as a compromise.
• The feasibility and appropriateness of using these broader limits for daily QA have not been established, posing a potential risk to accuracy.

Purpose of the Study:

• To analyze daily QA results using Statistical Process Control (SPC).
• To determine institution-specific tolerance limits for daily QA processes.
• To demonstrate the utility of SPC for optimizing QA procedures.

Main Methods:

• Analysis of three years of daily QA data using SPC.
• Calculation of process capability indices (Cp, Cpk, Cpm, Cpmk) with TG-142 limits (±3% for output, ±1% for flatness/symmetry).
• Establishment of customized tolerance and action limits, and comparison of control charts.

Main Results:

• Process capability indices for output generally met requirements (≥1), but some symmetry processes fell below ( <1).
• Customized limits varied significantly across different processes and staff.
• Existing TG-142 limits are often unsuitable for daily QA, necessitating SPC-derived limits.

Conclusions:

• SPC is a valuable and necessary method for establishing appropriate daily QA tolerance limits.
• Institution-specific limits derived from SPC enhance the reliability and accuracy of daily QA procedures.
• The findings highlight the inadequacy of applying broader QA limits to daily tasks.